The present invention relates to a fixing apparatus for use with an image forming apparatus.
A prior art fixing apparatus adapted for an image forming apparatus such as an electrophotographic copying apparatus, a facsimile apparatus and a laser printer usually includes a fixing roller which is provided with a rubber layer thereon and a heater thereinside and is pressed by a pressure roller. These rollers serve for permanently fixing toner images to a transfer sheet at the nipping position. In this type of fixing apparatus, a leading end portion of the sheet with the fixed toner images is apt to stick to the periphery of the fixing roller. Therefore, it has been customary to employ sheet separator pawls resiliently engageable at their free ends with the periphery of the fixing roller, for forcibly separating a leading end portion of the sheet from the periphery of the fixing roller.
A problem has existed in such a type of fixing apparatus in that an anti-offset layer made from an anti-offset material such as fluorine-contained polymers which is coated on the surface of the fixing roller is scraped by the free ends of the sheet separator pawls as the fixing roller rotates. Also, tubular metal shafts or sleeves have appeared. The result is that a sheet cannot be separated easily from the fixing roller and jamming of the sheet has happened. When it has happened, the fixing roller is thus judged to have run down and must be exchanged. As a life span of a fixing roller is mainly decided according to the flaws imparted on the periphery of the fixing roller by the sheet separator pawls, it is very important in order to get a high-reliability and a long span of life of the fixing roller for an electrophotographic copying machine or the like. how to reduce the damage to the surface of the anti-offset layer by the sheet separator pawls.
Some different approaches have been proposed to eliminate such an ocurrence. One approach is providing that the paper separator pawls go back and forth in the direction of an axis of the fixing roller and the movement of the pawls is synchronized with rotating of the fixing roller, which is disclosed by Japanese Laid-Open Patent Publication No. 52-39148. Another approach is proving that the paper separator pawls can be movable and be set in some predetermined positions by a person, which is disclosed by Japanese Laid-Open Patent Publication No. 62-1778. The position of the pawls are changed by the person when he observes the surface of the roller and judges that the roller should be changed because of damage thereto.
However, a problem with the device of the first approach is that the fixing roller cannot rotate at the correct speed because of the load of the moving pawls. As the pawls are leaning toward the moving direction and an engaging portion of the pawls pushes the surface of the fixing roller while the pawls moving, extra loads are loaded on the rotating roller. And as the fixing roller cannot rotate at the predetermined speed, the fixing condition is changed slightly. And a problem with the second approach is that it is difficult for people in general to judge the time when the fixing roller should be changed according to the flaws of the surface of the roller. Therefore, people tend to do nothing, with the result that the paper jams. And then they call a service person.